1. Field of the Invention
The present invention relates to a process for producing epitaxially coated semiconductor wafers having low-oxygen zone formed by epitaxy of monocrystalline substrate wafers in a single-wafer reactor. The invention furthermore relates to semiconductor wafers produced by this process.
2. The Prior Art
Semiconductor wafers, in particular composed silicon, which are used to produce electronic components and which are produced by sawing up rod-shaped or block-shaped workpieces, have as a rule oxygen concentrations of up to 9.times.10.sup.17 atoms/cm.sup.3. This oxygen content, which is introduced into the semiconductor material in a process-related manner by the use of quartz workpieces, for example, the crucibles used for Czochralski crystal pulling, is thoroughly desirable because it presents, as a lattice defect, nucleation centers for further impurities in the crystal lattice. This purification effect, known as "internal gettering," as a result of concentrating the residual impurities, is, however, only useful in the interior of the semiconductor wafer. In the surface region of the silicon wafers, in which the electronic components are integrated, these oxygen centers cause considerable disturbance. As a consequence of the process steps encountered in component production such as, for example, epitaxy, dopant treatment, oxidation and heat-treatment steps associated therewith, the oxygen centers have a tendency to precipitate formation and consequently cause stresses in the crystal lattice which result, in turn, in the failure of entire component groups.
It is therefore desirable to use wafers which contain an oxygen-depleted surface zone (so-called denuded zone) which is several .mu.m thick. Processes for producing a denuded zone have been known for a long time. Thus, the oxygen can be caused to diffuse out to the surface by heat-treating silicon wafers in a furnace (at temperatures of, for example, 1,000.degree. C.-1,200.degree. C.) under an inert gas atmosphere. After a time of five hours at a temperature of 1,000.degree. C., the layer thickness of the denuded zone achievable solely by diffusing out oxygen is more than 10 .mu.m (Huber, D.; Reffle, J.: Solid State Techn. 26(8), 1983, page 183).
The production of a denuded zone is advantageously carried out before, during or after the deposition, necessary as part of the component production, of a doped epitaxial layer because both operations require similar process temperatures and do not mutually impede one another. The thickness of the denuded zone produced depends essentially on the allowed diffusion times.
The deposition of an epitaxial layer serves, as a rule, to produce a sharp transition in the electrical properties of the semiconductor material. Normally, a steep increase in the resistance profile is desirable at the junction between the substrate and the epitaxial layer near the surface. Substrate and epitaxial layers are therefore normally differently doped.
In accordance with this prior art, the denuding step was hitherto carried out in a batch epitaxial reactor. Those are reactors having a capacity to receive more than one wafer, as a rule 4-50 wafers. As a result of the time required to deposit the epitaxial layer (growth rates in this type of reactor: 1 .mu.m to 2 .mu.m per minute), these reactors have sufficiently long cycle times to also enable the formation of a denuded zone meeting the specifications required hitherto--the thickness of the low-oxygen layer near the surface to be maintained should be at least 5 .mu.m. However, the expectations relating to the quality and extent of the denuded zone imposed by the component producers have risen. In order to satisfy these requirements, increasing residence times of the wafers in the batch reactor have to be accepted, as a result of which the process becomes increasingly uneconomical.
In addition, batch reactors have some disadvantages which have resulted in the fact that so-called single-wafer reactors are now preferably used for epitaxial processes. As a consequence of the long processing time of the wafers in the batch reactor, dopants diffuse out of the substrate and are partly incorporated in the growing epitaxial layer. Under these circumstances, the desired steep resistance profile at the junction between substrate material and epitaxial layer is reduced in an unacceptable manner (autodoping effect). Similar difficulties occur as a result of the fact that dopants systematically introduced in the course of a sequence of processing steps can virtually not be removed completely from the relatively large volume of such reactors and are later absorbed in the epitaxial layer in an undesirable manner (memory effect). The large wafers having diameters of 150 mm to 200 mm and above, which have to be processed to an increasing extent, exacerbate these problems and increasingly make the use of conventional batch reactors additionally more unattractive.
Single-wafer reactors have higher growth rates (5 .mu.m/minute and higher) and, consequently, make possible the epitaxy of even large wafers in shorter periods of time, the problems described and occurring with the use of batch reactors being insignificant.
A disadvantage of using single-wafer reactors is, however, that the residence time of the wafer in the reactor is insufficient to achieve the required extent of the denuded zone exclusively by heat treatment. Although the extent of the denuded zone can be increased to an adequate extent by additional heat treatment, this reduces the cost effectiveness of the process considerably, in particular in the case of high substrate oxygen content. The use of low-oxygen substrate wafers is, however, ruled out because the internal gettering described at the outset is only inadequately effective in such material.